# Elastic Collision

- Author:
- Luciano Troilo

In this lab you will calculate velocities, momenta, and kinetic energies to determine to what extent these quantities are conserved.
Principles: If no net external force works on a body (or a system of bodies), then that body’s momentum is constant – it does not change. In this instance, we say that the momentum is conserved. We refer to this as the law of conservation of momentum.
The momentum of an individual object may change, but the total for the system does not. Suppose that object 1 is moving with an initial velocity v1i, and collides with object 2, which is moving with an initial velocity v2i: the total momentum of the system BEFORE the collision is then
Pbefore = m1v1i + m2v2i
AFTER the collision, the objects will likely have new velocities v1f and v2f.
The total momentum after the collision is then
p after = m1v1f + m2v2f
If there is no net external force on the system, then
pbefore = pafter
Your job in this lab is to see if this equation holds true.
Another quantity often looked at in collisions is kinetic energy. The kinetic energy of a body is one-half the product of the mass and the square of the velocity:
KE = ½ mv2
Kinetic energy, unlike momentum, is a scalar quantity, and always positive. And the kinetic energy of a system of bodies equals the sum of all the individual kinetic energies.
A collision in which the total kinetic energy is constant before and after the collision is called a perfectly elastic or completely elastic collision. Such, of course, do not exist in the real world.

Procedure

- Set the masses of the two balls to be equal (0.5 kg is a good value).

- Set the length of pendulum to 2m

- Set the h1 of ball2 (red) at 2 m.

- Run the collision and note the velocities of each ball before and after the collision.

- Record these values on the data sheet. (Note that the subscript i means initial, or before the collision, and subscript f means final, or after the collision.)

- Reset the applet and change the mass of the red ball to 1kg. Repeat the collision and record the velocities.

- Reset the applet. Set the red ball to 0.1 kg and and the mass of green ball to 1kg. Repeat the collision and record the masses and velocities.

- Repeat the experiment changing the values of m1, m2, l and h1 at your choice.